As an example of the gas combustion type impact tool, a combustion gas driven nailing machine is known, which is adapted to inject a combustible gas into a sealed combustion chamber and form a gaseous mixture of the combustible gas and air therein, burn the gaseous mixture in the combustion chamber, generate a high-pressure combustion gas therein, exert the high-pressure combustion gas on a driving piston held in a driving cylinder, drive the driving piston with an impact thereon to strike a nail into a steel plate and concrete by a driver coupled to a lower surface of the driving piston. In such a combustion gas driving nailing machine, a container, such as a gas bottle filled with a combustible gas, is fixed in the machine. A battery is used as an electric power source for igniting the combustible gas. The combustion gas driving nailing machine is thus formed as a portable machine. This enables a nail and a pin driving operation to be carried out without being restricted by a power supply source, such as an electric power source and a compressed air supply source.
In the combustion gas driving nailing machine, a cylinder slidably holding a driving piston therein is provided in the housing. On the lower surface of the driving piston, a nail striking driver is connected. The driver is held and guided in a nail discharge port formed in a nose coupled to a lower portion of the housing. When the driving piston is driven in the driving cylinder, the driver joined to the driving piston is driven with an impact in the nail discharge port. A nail supplied to the interior of the nail discharge port of the nose is driven from the nail discharge port toward a work provided at a free end of the nose.
At an upper portion of the driving cylinder, an annular combustion chamber is formed. This combustion chamber is defined by an annular sleeve forming a circumferential wall of the combustion chamber, an upper wall formed by an upper housing, and an upper end surface of the driving piston. A combustion gas formed in this combustion chamber works on the driving piston, so that the driving piston is driven in the driving cylinder. In the combustion chamber, an injection nozzle for injecting a combustible gas put in a gas container, such as a cartridge, is formed so that the injection nozzle is opened therein. A rotary fan is used for generating a gaseous mixture of a predetermined air/fuel ratio by mixing the combustible gas injected into the combustion chamber with the air therein is further formed. The rotary fan is rotated by an electric motor, and the combustible gas injected into the combustion chamber and the air existing in advance therein are agitated, the gaseous mixture being thereby formed in the combustion chamber.
The combustion chamber is further provided therein with an ignition device used to ignite the gaseous mixture generated in the combustion chamber, and burn the gaseous mixture explosively therein. The ignition device is usually made of an ignition plug and the like for generating sparks by discharging a high voltage. When an operator actuates a trigger formed at a base portion of a grip formed so as to extend in the rearward direction of the housing so as to be integral therewith, the ignition device is operated to generate sparks in the combustion chamber. As a result, the gaseous mixture in the combustion chamber is ignited, and the nailing machine is driven (refer to JP-B-03-025307).
As described above, a gas combustion type impact tool in the related art may have a large air flow generated in the combustion chamber by a fan rotated by an electric motor. The air flow and the combustible gas may be injected via the injection nozzle, and the combustible gas and the air in the combustion chamber may be agitated in the whole region of the combustion chamber, a gaseous mixture being thereby formed. Therefore, the mixing of the combustible gas and air in the combustion chamber cannot be efficiently carried out, and it takes much time before the air/fuel ratio of the gaseous mixture in the whole region of the interior of the combustion chamber attains a level at which the condition in which the gaseous mixture can be ignited by the sparks generated by the ignition device is obtained. As a result, when the sparks are generated from the ignition device by operating the trigger immediately after the formation of the gaseous mixture is started with the combustible gas being supplied into the combustion chamber, the combustion of the gaseous mixture is not carried out. When it takes much time to form the gaseous mixture, an operation response is low, so that the operation efficiency is spoiled.